Cathode for electron discharge devices



Patented Jan. 17, 1939 PATENT OFFICE 2,144,249 CATHODE FOR ELECTRONDISCHARGE DEVICES Victor 0. Allen, Madison, N. J., assignor, by mesne Iassignments, to Radio Corporation of America,

. New York, N. Y., a corporation of Delaware No Drawing. ApplicationFebruary 23, 1935,

Serial No. 7,853

'7 Claims.

My inventionrelates to electron discharge devices, more particularly toimprovements in thermionic electron emitting cathodes for such devices.

The present application relates to modifications of the inventiondescribed and claimed in copending United States patent application,Serial No. 43,714, filed October 5, 1935, by Victor 0. Allen and JosephJohnson as joint inventors, and assigned to the same assignee asthepresent invention.

In conventional electron discharge devices with the filament type ofthermionic cathode commercial use has been made of pure tungstenfilaments,

activated thoriated' tungsten filaments, and oxide coated nickelfilaments. The pure tungsten filament cathode is a very stable emitterand is eflicient but must be operated at very high-temperatures toobtain ample emission. The activated thoriated tungsten cathode, such asdescribed in U. S. patent to Langmuir 1,244,216,

may be operated at lower temperatures than a pure tungsten filament andis more efficient, but is easily poisoned by oxygen and other gaseswhich adversely affect the emission, is not very stable when operated"at high voltages, and requires special processing and seasoning. It hasalso been found that when operated at ultra' high frequencies, forexample above 15 or 16 megacycles, that emission becomes erratic andthat there is sometimes a total loss-of emission. In addition, theactivated thoriated filament is apt to be brittle, and if operated attoo high a temperature quickly deactivates and loses its ability to emita suflicient numberof electrons to be useful. The oxide coatedniokelcathode operates at a much lower temperature than either kind oftungsten cathode and is efiicient, but cannot be satisfactorily operatedin tubes in which very high plate voltages, such as 1000 volts, areused, and requires considerable processing and seasoning.

Thoriated molybdenum has also been thoroughly investigated as a filamentcathode emitter, but has proved to be rather unstable, quite sen .sitiveto poisoning by gas within the tube, and

must be operated at fairlyhigh temperatures, between 1400 and 1600 C. toprovide satisfactory emission. For these and other reasons thoriatedmolybdenum has not come into commercial use as a cathode filamentmaterial.

In the conventional high vacuum or gas tube operating at high voltages,only filamentary type cathodes have heretofore been used because theoxide coatings of the indirectly heated cathodes quickly disintegrate athigh voltages, and without these coatings no material was availablewhich had sufiicient emission unless raised to very high temperatureswhich cannot be practically generated with the conventional indirectlyheated cathode construction,

An object of my invention is to provide a duo-' tile thermionic cathodewhich is a stable, efficient and long lived emitter, at desirableoperating temperatures. A further object is to provide an eflicientthermionic cathode which can be satisfactorily used at high frequenciesand high voltages and requires little seasoning or'processing. Anotherobject of my invention is to provide'an indirectly heated cathode forhigh voltage gas or vacuum tubes.

In accordance with my invention I use a refractory metal having a highmelting point, preferably 2400 C, or higher combined with thoria andchromium or their equivalents. Of these refractory metals I prefer touse tungsten, molybdenum or tantalum. The thoria may be incorporated inthe tungsten metal to form a base, and the chromium plated on thethoriated tungsten base, or the pure tungsten, molybdenum or tantalumbase may be chromium plated and the thoria applied as a coating on theplated base.

The thoriated tungsten base may be prepared as a ductile metal inaccordance with U. S. Patent I 1,082,933 to W. D. Coolidge, or. bysquirting the finely divided material admixedwith a binder. For example,if tungsten is used I may add powdered nitrate of thoria. to thepowdered oxide of tungsten before the reduction of the oxide oftungsten, or I may add thoria to the oxide of tungsten after reductionbut before consolidation of the metal powder by sintering and mechanicalworking to the solid state as described in the said Coolidge patent. Theamount of thoria may vary from about /2 of 1% by weight of the metal toabout 3%0f the metal. Tungsten-containing more than 2% or 3% of thoriais eX- tremely brittle and hard to work, but in general I have found itdesirable for best results to add as much thoria as may be tolerated inthe refractory metal without interfering with the subsequent swaging anddrawing of the metal. This has been found in practice to be up to 1 fortungsten. A filament of this thoriated metal may be used. In generalthelarger the percentages of thoria the better the emitter. While thoriais preferred, equivalents of thoria, for eX-.

yttrium and lanthanum may be used with tungsten to form a base and thebase then chromium plated in a conventional chromium plating bath,

. the essential constituents of which are chromic base may vary betweenwide limits, for example,

from a few hundreths of an ampere to over 1 ampere per squarecentimeter, for best results a current density of about 1 ampere persquare centimeter for a period of one minute seems to produce the mostsatisfactory results. The resulting chromium plated thoriated wire'maybe used for a thermionic electron emitting cathode without furthertreatment, but for best results in the case of the thoriated tungstenbase I prefer to flash theplated filament in vacuum at'over voltage; forexample, to heat the filament to a temperature of approximately 2600 K.for about thirty seconds, and thereby facilitate the activation of thefilament. It is also desirable, though not necessary, to insurestability of emission to season the filament by operating the electrondischarge device containing the filament with normal voltages appliedfor approximately fifteen minutes in an oscillating circuit.

A cathode made in accordance with my invention has a normal operatingtemperature several hundred degrees K. below the normal operatingtemperature of the conventional activated thoriated tungsten cathode,and in the conventional type of tube operated at the customary platevoltage the same plate current can be obtained from my improved cathodeat its normal operating temperature with only about one-half the energyper unit area that must be used to obtain the same plate current fromthe conventional activated thoriated tungsten cathode at its normaloperating temperature.

.While I do not wish to be limited to any particular theory it isbelieved that the chromium acts as a catalyst to facilitate the rapiddiffusion of the electron emitting metal, such as thorium, to thesurface of the filament. for producing copious emission.

A pure tungsten, molybdenum, or tantalum filament may be chromium platedas above described and then brushedwith thoria powder suspended inacetone and of a particle size which will pass thru a 200 mesh screen orsmaller. This will also provide a very efiicient emitter. The thicknessof the coating should come within the limits described above. Treatmentto insure stability of emission as described above is desirable but notnecessary.

When the cathodes are to be used in low voltage tubes, for example under1000 volts, to increase the electron emissivity any of the cathodesabove described may be coated with the usual electron emitting coatingof alkaline earth metal oxides, such as barium oxide and strontiumoxide, which may be applied in a conventional manner by applying bariumand strontium carbonates and converting them into the oxides.

A cathode made in accordance with my invention is a very eflicientemitter. It will operate at a comparatively low temperature (1200 K.)and very high voltages can be used on the plate of a tube using mycathode up to approximately 3000 volts without the cathode suffering anyill efiects due to ion bombardment. These last two characteristics makemy invention particularly suitable for indirectly heated cathodes inhigh voltage gas or vacuum tubes.

Cathodes made according to my invention are very satisfactory in tubesused for generating very high frequencies at high voltages and whileseasoning improves the initial stability and operating characteristicsit is not essential in order to provide a cathode which is a goodemitter and operates satisfactorily. The resulting cathode is a Verystable emitter and is not easily poisoned by as. The cathode readilyrecovers its emitting characteristics even if accidentally operatedtemporarily at too high temperatures. ionization will not destroy thetube due to ion bombardment, and the cathode will recover its emissionif temporarily impaired by being subjected to high voltages.

While I have indicated the preferred embodiment of my inventionof whichI am now aware and have also indicated only certain specific applicationfor which my invention may be employed, it will be apparent thatmyinvention is by no means limited to the exact forms illustrated or theuse indicated, but that many variations may be made in the particularstructure used and the purpose for which it is employed withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim as new is:

1. A thermionic cathode adapted for use as an electron emitter inelectron discharge devices comprising a refractory metal body consistingpredominantly of tungsten, and the remainder containing thoria andelectrolytically applied chromium.

2. A thermionic cathode for use in an electron discharge device andconsisting essentialy of one of the group of metals tungsten andtantalum, in

combination with electrolytically applied chromium and at least one ofthe group of substances consisting of thoria, zirconium, uranium,cerium,

titanium, vanadium, yttrium and lanthanum.

3. A thermionic cathode for use in an electron discharge device andconsisting essentially of tungsten and containing thoria and a coatingof electrolytically applied chromium on the thoriated tungsten.

, 4. A thermionic cathode for use in an electron discharge device andconsisting essentially of one of the group of metals, tantalum, tungstenor molybdenum and a coating of thoria and elec trolytically appliedchromium on said metal.

5. A thermionic cathode for use in an electron discharge device and.comprising a metal of high melting point, a chromium plating on saidmetal and a coating of thoria on said chromium plating.

6. A thermionic cathode for use in an electron discharge device andcomprising molybdenum, a chromium plating on said molybdenum and acoating of thoria on said chromium plating.

7. A thermionic cathode for use in an electron discharge device andcomprising tungsten, a chromium plating on said tungsten and a coatingof thoria on said chromium plating.

- VICTOR O. ALLEN.

High voltage CERTIFICATE OF CORRECTION. Patent No. 2,1hl 2u9.

January 17, 1959. VICTOR o. ALLEN.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,sec- 0nd column, line 2, for (1200 IL) read (1200 0.); and, that thesaid Letters Patent should be read with this correction therein that thesame may conform to the record of the ca se in the Patent Office. Signedand sealed this 18th day of July,- A. D. 1.959.

Henry Van Arsdale, Acting Commissioner of Patents.

(Seal) I

